The optical fiber amplifier, especially Erbium-doped optical fiber amplifier (EDFA), has already become one of the key parts of optical communication systems. EDFA has given great impetus to the development of wavelength division multiplexing systems since it came into being. However, as WDM systems increase in capacity and rate, the gain unflatness in EDFA is obviously affecting the system performance.
Generally, there are two methods to flatten the gain in EDFA. One is to add a gain flattening filter (GFF) to EDFA. The other is to change the basic material of Erbium-doped optical fiber or to add other materials to the Erbium-doped optical fiber. But generally, before the amplifier is designed, the basic and doped material of Erbium-doped optical fiber has been predetermined, so the gain flattening filter (GFF) is generally used for flattening the gain in the design.
The gain flattening filter (GFF) can be placed in a flexible position. It can be placed before or after all of the gain stages. If there are two or more gain stages available, it can be put between any two gain stages; even several gain flattening filters (GFF) can be placed respectively. However, no matter where the gain flattening filter is placed, the spectrum shape of gain flattening filter (GFF) is designed so as to achieve a good performance of gain flatness.
However, the existing test methods are only restricted to GFF spectrum shape under a given length of optical doped fiber and a specific level of population inversion.